An Austin native who learned to swim in Barton Springs, Gilbert was the 1946 and 1947 high school state champion and record holder in the 100 freestyle. He won the 1948 AAU national championship in the 200 freestyle, earning a spot on the 1948 Olympic team and was on the 800 freestyle relay that won the gold medal in London. Gilbert was a Southwest Conference champion and all-American in each of his three seasons at the University of Texas.